1. Field of the Invention
The present invention relates to a strip tension control apparatus for controlling the tension of a strip by threading the strip between a transportation roll and a movable-transportation roll and moving the movable transportation roll. The apparatus is adapted for maintaining a given strip tension in a process line for rolling or the like.
2. Description of the Related Art
In order to secure reliable quality of a strip in a process line for metal or nonmetal rolling or the like, it is necessary, in general, to perform a continuous operation in the central section of the line while transporting the strip at a fixed speed and applying a tension to the strip.
In the supply- or delivery-side section of the process line, limited-length strips are wound off or up in the form of coils. At breaks in the coil jointing or at the time of recoiler change, each strip is accelerated, decelerated or stopped supply- or delivery-side section.
In order to secure continuous operation in the central section despite such transitory acceleration, deceleration or stopping in the supply- or delivery-side section, the process line is provided with a looper.
When the looper operates as the strips are decelerated, stopped, or accelerated in the supply- or delivery-side section, however, a variation in tension may be transmitted from the supply- or delivery-side section to the strips in the continuously running central section. This transmission of the variation in tension adversely effects the quality of the strip in the central section and causes the strips to meander, thus possibly breaking the strips.
To cope with this, a tension control apparatus has been proposed in Japanese Patent Laid-Open No. 1-308347. The prior art apparatus includes a dancer roll disposed in the central section, whereby the transmission of the variation in tension is deterred to apply a fixed tension to the strips.
The prior art tension control apparatus having the dancer roll is constructed in the manner shown in FIG. 4. In FIG. 4, a strip 1 is passed from one transportation roll 2 to the other transportation roll 2 via a dancer roll 3. The dancer roll 3 is linked to a wind-up drum 4 and a counterweight 6 by means of a wire 5, and the drum 4 is connected to a motor 8 through a speed reducer 7. The motor 8 causes the speed reducer 7 to rotate the wind-up drum 4, thereby moving the dancer roll 3 up and down. The tension of the strip 1 is controlled by regulating the torque of the motor. Guide means 9 is used to fix the direction of action of the dancer roll 3.
However, the conventional prior art tension control apparatus having the dancer roll is helpless against a drastic external variation in tension of the strip in the central section. In operation, high mechanical resistances are produced between the dancer roll 3 and the guide means 9 and between the wind-up drum 4 and the wire 5.
The dancer roll 3 is subject to a high moment of inertia during the operation caused by the action of the wind-up roll 4, the motor means 8, and the speed reducer 7, as shown in FIG. 4.
A backlash of the speed reducer results in a delay in operation or a new variation in tension attributable to the action of the dancer roll.
Furthermore, the conventional tension control apparatus having the dancer roll is quite helpless against a fine variation in tension due to its great structural mechanical loss, backlash in its mechanical system, and high mechanical resistance. Thus, the prior art does not permit high-accuracy tension control in response to variations in tension in a continuous operation of the type described above.
Modern steel sheets for use in automobiles and the like are expected to respond quickly to a fine variation in tension, since they are made of very-low-carbon steel, have a small sectional area, and are transported at a super-high speed, as high as 1,000 m/min, as they are processed. There is, therefore, a demonstrated need for advancement in the art of continuous operation strip tension control.